Polyethylene is divided into high-density (HDPE, density 0.94 g/cc or greater), medium-density (MDPE, density from 0.93 to 0.94 g/cc), low-density (LDPE, density from 0.91 to 0.93 g/cc), and linear low density polyethylene (LLDPE, density from 0.91 to 0.93 g/cc). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials. One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, shipping sacks, food packaging films, multi-wall bag liners, produce bags, deli wrap, stretch wrap and shrink wrap. The key physical parameters of polyethylene film include tear strength, impact strength, tensile strength, stiffness and clarity. Tear strength is measured in machine direction (MD) and transverse direction (TD).
LLDPE film has higher impact strength and MD tear, while HDPE has higher stiffness and tensile strength. When LLDPE producers attempt to increase the density (thereby increasing the modulus of the film), they often encounter losses in impact strength and MD tear. Historically, blending LLDPE and HDPE has not achieved “breakthrough” success. The blends often give films that have improved stiffness and tensile properties, but the impact and tear properties are usually sacrificed. Most LLDPE resins are processed on conventional extruders with dual lip air rings and run “in the pocket” or with no or small stalks, while HMW-HDPE or HMW-MDPE is processed on extruders equipped with a single lip air ring and run with neck heights between 6 and 10 times the die diameter. LLDPE-rich blends are difficult to process on HDPE film extruders in high stalk applications due to their poor bubble stability.
It would be desirable to prepare polyethylene thin films from LLDPE-rich blends comprising LLDPE and HMW-HDPE or HMW-MDPE. Ideally, the blends could be processed on a single lip air ring at a neck height of 6 to 10 die diameters. More particularly, the film would have increased tear strength.